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stm32/i2c: Fix I2C frequency calc so it doesn't exceed requested rate.

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Prior to this commit, the actual I2C frequency can be faster than specified
one and it may exceed the I2C's specification for Fast Mode.  The frequency
of SCL should be less than or equal to 400KHz in Fast Mode.

This commit fixes this issue for F4 MCUs by rounding up the division in the
frequency calculation.
This commit is contained in:
yn386
2022-09-19 10:14:27 +09:00
committed by Damien George
parent 65d82066a8
commit a74e4fabeb
3 changed files with 11 additions and 2 deletions
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5
ports/stm32/i2c.c
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@@ -70,10 +70,11 @@ int i2c_init(i2c_t *i2c, mp_hal_pin_obj_t scl, mp_hal_pin_obj_t sda, uint32_t fr
// SM: MAX(4, PCLK1 / (F * 2))
// FM, 16:9 duty: 0xc000 | MAX(1, (PCLK1 / (F * (16 + 9))))
// (the PCLK1-1 and +1 at the end is to round the division up)
if (freq <= 100000) {
i2c->CCR = MAX(4, PCLK1 / (freq * 2));
i2c->CCR = MAX(4, ((PCLK1 - 1) / (freq * 2) + 1));
} else {
i2c->CCR = 0xc000 | MAX(1, PCLK1 / (freq * 25));
i2c->CCR = 0xc000 | MAX(1, ((PCLK1 - 1) / (freq * 25) + 1));
}
// SM: 1000ns / (1/PCLK1) + 1 = PCLK1 * 1e-6 + 1